Use of video recorders for recording and storing television programs on magnetic tape or other storage medium has become increasingly widespread. In the past, it has been necessary for an operator to manually stop and restart the recording operation in order to prevent the recording of undesired advertising or commercial messages along with the television desired to be recorded.
Generally, a blank or dark picture break of about one-half to three seconds immediately precedes and follows each commercial. Also, most commercials haye a standard duration, one of the most common being a duration of thirty seconds. These facts are utilized in the above-referenced U.S. patent application, Ser. No. 038,161, which describes an editing system for use with a video tape or cassette recorder which monitors the video signal being taped and automatically interrupts the taping operation for an predetermined time upon sensing a decrease in the audio signal indicating the absence of picture information in the video signal for a period of time corresponding with at least one frame or complete picture, which occurs at the beginning of the blank or dark picture break preceding a commercial. Thus, by setting the pause or interruption period in the range of thirty-five or forty seconds so that this pause period includes the blank or dark picture break occurring immediately after a thirty second commercial, the editing system automatically stops the taping operation immediately before a thirty second commercial, and restarts the taping operation a few seconds after the thirty second commercial.
However, many commercials are of a standard duration greater than thirty seconds. For example, sixty second and ninety second commercials are very common in commercial television broadcasting. Thus, upon the occurance of one of these longer commercials, the editing system described in U.S. patent application Ser. No. 038,161, not only will fail to edit the entire commercial, but will also sense the blank picture break immediately following this commercial and consequently edit a thirty-five to forty second portion of the program desired to be taped. Also, a dark or blank picture break lasting for at least one complete frame or picture, which is an integral part of the television program desired to be taped, or which occurs as a result of a momentary malfunction of the video signal transmitter, will also result in the editing of a thirty-five to forty second portion of the program desired to be taped.
Further, in the editing system described in the the above-referenced U.S. patent application Ser. No. 038,161, the video signal being taped is rectified, amplified, and filtered to produce a first DC voltage signal proportional to an average video signal voltage during a period corresponding to at least one complete frame. This first DC voltage signal is compared with a reference DC voltage whose voltage level is manually adjusted to be slightly greater than the voltage of the first DC voltage in the absence of picture information in the video signal for a period of time corresponding to at least one complete frame. At any particular video receiver location, this reference voltage must be adjusted for the video signal of each television station whose programs are desired to be taped. Thus, in order to facilitate recording programs of several television stations, it is highly desirable that this reference DC voltage be automatically calibrated for the video signal of each television station.